1. Field of the Invention
The present invention relates to a liquid developer suitable for use in electrostatic printing.
2. Description of the Related Art
“Electrostatic printing” is a term used to describe various non-impact printing methods, which enables forming a visible image by attracting charged particles for forming an image to charging sites on a substrate.
The charging site enables forming an electrostatic image, which may be referred to as a “latent image,” thereon. The electrostatic image is formed temporarily retained in on a photoconductor or a pure dielectric. The electrostatic image may be visualized thereon, or may be transferred onto another substrate, and then visualized so as to develop.
Additionally, the charging site may be the reflection of those structured charges existing within a permanently polarized material, as is the case with ferroelectrics and other electrets.
Electrostatography encompasses those processes normally known as electrophotography and electrography.
In general, a liquid developer for electrostatography is prepared by dispersing an inorganic or organic coloring agent such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink, and the like, into a liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylic resins, alkyd resins, rosins, rosin esters (ester gums), epoxy resins, poly(vinyl acetate), styrene-butadiene polymers, or the like.
Additionally, to generate or enhance the electrostatic charge on such dispersed polymer particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids, lecithin, organic phosphorus compounds, succinimides, sulphosuccinates, or the like.
In such liquid developers, whether positively or negatively charged, there is one ingredient of common generic character, namely a carrier liquid.
Since the beginning of the history of liquid toners, it has been recognized that certain electrical properties of the carrier liquid are mandatory requirements for the effective functioning of a conventional electrostatographic liquid development process.
These are low electrical conductivity and other requirements became obvious, such as the needs for low toxicity, increased fire safety, low solvent power, and low odor.
For these reasons, isoparaffinic-hydrocarbons such as the Isopar range manufactured by Exxon Mobile Corporation, the Shellsol range manufactured by Shell Chemical, Co., Ltd., and the Soltrol range manufactured by Phillips Petroleum, Co., Ltd. became the industrial standards for liquid toner carriers.
In more recent times, however, certain deficiencies in these isoparaffins have become apparent. Environmental concerns have given a liquid development process to reduce or eliminate volatile emissions, under increasing pressure. Flammability has also become important, regarding the more stringent transport regulations worldwide.
New designs of image fusing sites are exposed to increased importance on the thermal stability of carrier liquids.
In order to overcome these limitations and restrictions, it was found out that silicone fluids were the most effective materials that have the desirable properties of a carrier liquid for a liquid developer of past and current.
Silicone fluids have been mentioned in the context of liquid developers, for example, in the U.S. Pat. No. 3,105,821 of S. W. Johnson's, and the U.S. Pat. No. 3,053,688 of H. G. Greig's. Both of these early patents recognized the values and strength of silicone fluids. However, these two patents see the functions of the liquid developer relatively experimentally. These patents simply describe the mechanical dispersion of dry toners into the silicone fluid, with no regard to chemical compatibility. These patents in turn determine the final particle size and stability of the dispersion thus produced.
Recently, silicone fluids have again raised the attention, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 03-43749.
However, the JP-A No. 03-43749 discloses solely the mechanical dispersion. It does not state that the necessity of the mechanical dispersion for chemical compatibility, which is fairly established in a field of a developer, nor it states a charge control agent, which is the most important feature.
It is well known that silicone fluids is less soluble to plastics and that this property is well suited to prolong copy machine components and an organic photoconductor. This property produces an unfavorable result that a large number of polymers, which is usually utilized in a liquid developer, is insoluble in or incompatible to silicone, whether or not the polymers are controlled based on the U.S. Pat. No. 3,990,980 of G. Kosel et. al, or based on the U.S. Pat. No. 5,112,716 of Kato et. al, which is more recent application, or whether or not the polymers are chemically controlled based on an ordinary dispersion disclosed in the JP-A No. 03-43749.
Insolubility or incompatibility of the polymers causes a problem that the particle size and the stability of the dispersions thus prepared are limited and hence that it prevents reallogation of the dispersion because the polymers are not absorbed in the dispersed coloring agent after dissolved in silicone.
A demand has been made on a stable liquid developer that meets environmental requirements and enables image-forming properties such as color, tone, resolution, or the like. The JP-A No. 08-505709 and JP-A No. 08-505710 each disclose a liquid developer containing an unadulterated silicone as the carrier liquid.